Picked this up recently after realizing that it was the only game in the series that I didn't have:
nice. thats pretty cool, any fun?
I haven't played it that much since I got it but at first glance it is kind of disappointing compared to the other games. Everything feels cramped because of the tiny gameboy screen instead of wide open and Keen is somewhat hard to control. Once you get used to this there is also the fact that the one level I've played so far is a confusing mess, even with the arrows that tell you where to go it still usually isn't obvious where, exactly you're supposed to go or what you're supposed to do. The level also feels like it goes on for way too long.
There are also a couple of differences with the other games in this one, in the original games you had a jump and a pogo button and to fire you'd hit them both at once but in this one you get a jump and a fire button and to pogo you hit down and jump. I kind of like this compared to the originals. Another difference, although minor, is that you can't freely roam the world map, the arrows just take you on a predefined path between levels.